Epitaxial structures of low temperature grown gallium arsenide (GaAs) photoconductive devices can be tailored to the specific and differing needs of both free-space terahertz generation and free-space terahertz sampling. Low temperature GaAs can also be grown as a heterostructure with a tailored growth profile. When grown at approximately 600° C., GaAs has a recombination rate on the order of 1 nanosecond. Under these conditions, the strucutre is its natural or stoichiometric state, and has equal quantities of Ga and As forming the lattice.
It is also known that lowering the growth temperature to approximately 200° C. causes the As concentration to increase relative to Ga, forming a nonstoichiometric structure. A subsequent annealing step at the usual growth temperature of 600° C. for 10 minutes creates a form of single-crystal GaAs that possesses high resistively, high mobility, and subpicosecond carrier lifetime—all of which make such a structure well-suited to terahertz generation and sampling. Nevertheless, the performance of a free space terahertz generation by a low temperature GaAs photoconductive device can be enhanced by growing a lattice-matched heterostructure formed of low temperature GaAs and aluminum gallium arsenide (AlGaAs) on top of a normal GaAs substrate.
The benefits of the AlGaAs are numerous. For example, the barrier layer created by the AlGaAs confines the photogenerated carriers to the low temperature GaAs region. Generally, the incident light is absorbed in the low temperature GaAs region, but carrier diffusion can force both electrons and holes out of this region and into the substrate, where they recombine at a rate of approximately one nanosecond. The AlGaAs layer will prevent any carriers from thermalizing or tunneling to the GaAs substrate. Low temperature AlGaAs has a subpicosecond carrier lifetime similar to LT-GaAs. Additionally, the AlGaAs barrier layer reduces the dark and illuminated current of a biased device by removing the conduction path through the GaAs substrate layer. The increased resistivity allows a greater bias to be applied with reduced chance of damage by current or heat dissipation in the biased region.
Sampling of a free space terahertz waveform occurs when the illuminated photoconductive gate conducts for a time shorter than the entire terahertz wave cycle. During the conduction period, charge flows from one side to another of a dipole antenna structure due to the potential difference induced by the terahertz wave. The amount of current flow per sampling optical pulse is proportional to the terahertz voltage potential and the off-state resistance of the interaction area. The antenna structure has an inherent capacitance, and unless the illumination is near saturation to bring the resistance very low, the resistance-capacitance RC time constant is long enough that the sub-picosecond conduction period will not fully equilibrate the capacitance to bring the instantaneous antenna potential difference to zero.
Any amplifier circuit connected to the terahertz antenna must have very high impedance at terahertz frequencies. Non-ideal amplifiers with low impedance at terahertz frequencies will serve to equilibrate charge in the antenna in response to the terahertz field without the action of the photoconductive gate.
The combined off-state resistance and impedance of the amplifier with the capacitance of the antenna, leads, and amplifier yield an RC time constant which limits the rate at which the optical sampling gate can be swept through repeated identical terahertz waveforms. If this RC time constant is too large, the recorded terahertz waveform will be distorted in frequency response, phase and amplitude. Differing combined circuits can be chosen for the best signal to noise and RC time constant to meet the scanning rate. Existing designs have time constants limiting the terahertz waveform scanning to less than 10 Hz corresponding to electrical bandwidths of 100 Hz. For many applications it is desirable to scan at 10,000 to 1000 Hz corresponding to electrical bandwidths up to 1 MHz.
The photoconductive-gated current can either be amplified by measuring the current or the off state voltage of the THz antenna which has been (repetitively) charged by the photoconductive current. The voltage across the antenna is proportional to the charge divided by the capacitance of the antenna.
Existing terahertz receivers are generally current amplifiers. Current amplifiers typically have low impedance and therefore a minimal RC time constant. However, they are typically used at very slow scanning speeds to achieve adequate signal to noise. When the illumination of the photoconductive region is well below saturation, or when a terahertz field is small, the amplifier noise current and any off-state noise current may be large in comparison to the actual current induced by the terahertz field. The mismatch in amplifier impedance with the off-state photoconductive device impedance may yield low signal to noise ratios. Current amplification can be used when the active area is strongly illuminated and slow scanning speeds are used. These amplifiers are also ideal for systems where stray capacitance is present and slow scan speeds are acceptable.
The current state of the art employs a photoconductive gate/antenna assembly simply as a current source for an external amplifier. Integration and signal averaging occur after the external amplifier. At low signal levels, the noise floor of a current amplifier is sufficiently high limits the sensitivity of the device. Moreover, because current amplifiers are not integral to the photoconductive gate/antenna assembly, the connecting wires increased the inherent capacitance in the system and adversely affected the response time of the assembly. As such, there is a need in the art for an integrated photoconductive gate and amplifier assembly in which the response time is optimized and the signal to noise ratio is maximized.
Accordingly, the present invention includes an amplified photoconductive gate having an integrated antenna and voltage amplifier. Voltage amplifiers can yield superior signal to noise because their input impedance can be large and match the large impedance of the photoconductive sampling gate. When the illumination of the gate is well below saturation, the large amplifier impedance allows the antenna amplifier circuit capacitance to reach equilibrium as driven by the repeated terahertz potential on the antenna during the sample gate. This multi-sample integration allows the antenna voltage to rise well above that of a single sample. Furthermore, by reducing the stray capacitance inherent in cabling, equilibrium can be reached in a minimal number of illuminations, thus allowing rapid scanning of the signal.
Voltage amplifiers can be integrated into a single housing with the photoconductive antenna. Integrating a low-input capacitance amplifier with the terahertz receiver increases sampling speeds, lowers the cost, and improves product robustness over the use of a typical current preamplifier. The signal-to-noise ratio achieved with voltage amplifiers is equal to or better than that achieved by systems employing current amplifiers, while still gaining all of the noted benefits.